[Premium-Rx] use of precision time base

[David I. Emery]

On Tue, Dec 11, 2007 at 06:00:08PM -0600, David F. Reed wrote:
> My apologies up front for cross posting; these lists pose by best hope 
> for getting answers, so here goes...

	At the risk of suggesting cross posting, there is an ACTIVE list
with a lot of time and frequency hobbyists and quite a few professionals
in the field on it called time-nuts at febo.com.    Very much worth joining
if you have an interest in high accuracy time and frequency - those
folks have rubidiums, cesiums, a couple of home basement hydrogen
masers, and  lots and lots of GPS disciplined oscillators of various
levels of performance and design sophistication and probably as a group
constitute much of the public expertise on the subject on the net.

	The easiest solution for ham frequency and time standard use is
undoubtedly one of the eBay ex-telecom GPSDOs with a 10 MHz output, 1
PPS and ASCII serial port time of day.   The HP units and the Trimble
Thunderbolt family is a good place to start.   These units can certainly
supply frequency reference to parts in 10^11 or better and time of day
to maybe 25-50 NS using a rooftop mounted antenna (also cheap on Ebay).

	The only real advantage of rubidium based references is that
their intrinsic stability over time is such that they are often accurate
enough for many purposes if only calibrated occasionally (certainly
parts in 10^9 is possible this way and maybe a few in 10^10 in some
acases).   This high stability means that they can continue to work
without an external reference allowing operation in the absence of GPS. 
This is important in some professional telcom applications where GPS may
be jammed, or the GPS antenna or feedline damaged, or some national or
international crisis make GPS unavailable for a while - for ham
applications most of this is much less important and the primary reason
for using a rubidium is more likely to be that its accuracy is high
enough for the application with only infrequent calibration and
installing and maintaining a GPS antenna is inconvenient or impossible.

	Telcom grade rubidiums do tend to have less good short term
stability and phase noise than the best of quartz frequency standards,
so if those parameters are critical using a GPSDO based on a high
quality quartz oscillator may give better performance and may well have
plenty good enough short term holdover performance in the absence of GPS
to satisfy most needs (at least if there is a UPS powering the unit so
one is not dealing with a power outage and  cooling of the crystal).  
Telcom sites such as CDMA family base stations tend to use rubidium as
frequency and time backup because it give more hours to fix things if
lightning hits the GPS antenna or something cuts the cable than if they
were just high precision OCXO based - this extra time before they drift
out of sync allows cheaper and easier field repair operations which
justify the extra cost and power consumption.

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."